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Journal of Volcanology and Geothermal Research 341 (2017) 172–186

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Journal of Volcanology and Geothermal Research

journal homepage: www.elsevier.com/locate/jvolgeores

Sr- and Nd- variations along the San Pedro – volcanic chain, N. : Tracking the influence of the upper crustal -Puna Body

Benigno Godoy a,⁎,GerhardWörnerb,PetrusLeRouxc, Shanaka de Silva d, Miguel Ángel Parada a, Shoji Kojima e, Osvaldo González-Maurel f, Diego Morata a, Edmundo Polanco g,1, Paula Martínez a,2 a Centro de Excelencia en Geotermia de los (CEGA), Departamento de Geología, Facultad de Ciencas Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, , Chile b Abteilung Geochemie, GZG, Göttingen Universität, Goldschmidtstraße 1, Göttingen 37077, Germany c Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa d College of , Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA e Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, , Chile f Programa de Doctorado en Ciencias, Mención Geología. Departamento de Ciencias Geólogicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile g Energía Andina S.A. 5630, Las Condes, Santiago, Chile article info abstract

Article history: -related that erupted in the Central Andes during the past 10 Ma are strongly affected by Received 13 January 2017 crustal assimilation as revealed by an increase in 87Sr/86Sr isotope ratios with time that in turn are correlated Received in revised form 29 May 2017 with increased crustal thickening during the Andean . However, contamination is not uniform and can Accepted 29 May 2017 be strongly influenced locally by crustal composition, structure and thermal condition. This appears to be the Available online 31 May 2017 case along the NW-SE San Pedro - Linzor volcanic chain (SPLVC) in northern Chile, which straddles the boundary of a major zone of partial melt, the Altiplano_Puna Magma Body (APMB). Herein we report 40Ar/39Ar ages, com- Keywords: fl San Pedro – Linzor volcanic chain (SPLVC) positional and isotope data on from the SPLVC that track the in uence of this zone of on Altiplano-Puna Magma Body (APMB) erupted lavas with geochronological and geochemical data. Ages reported here indicate that SPLVC has evolved Isotopic shift in the last 2 M.y., similar to other volcanoes of the Western Cordillera (e.g. , , ). 87Sr/86Sr Geochronology ratios increase systematically along the chain from a minimum value of 0.7057 in San Pedro to a maxi- mum of 0.7093–0.7095 for the and Cerro de Leon dacites in the SE. These changes are interpreted to re- flect the increasing interaction of SPLVC parental magmas with partial melt within the APMB eastwards across the chain. The 87Sr/86Sr ratio and an antithetic trend in 143Nd/144Nd is therefore a proxy for the contribution of melt from the APMB beneath this volcanic chain. Similar 87Sr/86Sr increases and 143Nd/144Nd decreases are observed in other transects crossing the boundary of the APMB. Such trends can be recognized from NW to SE between , Ollagüe, and Uturuncu volca- noes, and from Lascar volcano to the N-S-trending Putana-- volcanic chain to the north. We interpret these isotopic trends as reflecting different degrees of interaction of mafic parental melts with the APMB. High 87Sr/86Sr, and low 143Nd/144Nd reveal zones where the APMB is thicker (~20 km) and more melt-dominated (~25% vol. partial melt) while lower 87Sr/86Sr, and higher 143Nd/144Nd reveal thinner marginal zones of the APMB where lower contents of partial melt (b10% vol) involves reduced interactions. The lowest Sr- isotope ratios, and higher Nd-isotope ratios (where available) occur in magmas erupted outside the APMB (e.g. San Pedro, Lascar and Aucanquilcha volcanoes), indicating a diminished influence of crustal partial melts on pa- rental mafic magmas. These geochemical parameters provide a useful tracer for the extent and significance of crustal partial melt bodies in magma genesis in the Central Andes. © 2017 Elsevier B.V. All rights reserved.

1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (B. Godoy). While it is known that most magma at subduction zones is generat- 1 Present address: Servicio Nacional de Geología y Minería, Avenida Santa María 0104, ed by flux melting in the asthenospheric wedge (Tatsumi et al., 1983; Providencia, Santiago, Chile. 2 Present address: Advanced Mining Technology Center, Avenida Tupper 2007, Grove et al., 2012), in continental magmatic arcs, the role of the Santiago, Chile. in controlling the evolution of even the most maficmagmasisclear

http://dx.doi.org/10.1016/j.jvolgeores.2017.05.030 0377-0273/© 2017 Elsevier B.V. All rights reserved. B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186 173

(e.g. Davidson et al., 1990). This is most obvious in arcs built on thick A dominant feature of the Neogene history of the Central Andes is such as the Central Volcanic Zone (CVZ) of the one the most extensive on Earth, the Neogene Cen- Andes. Arc migration and crustal thickening to 70 km in the Central tral Andean Ignimbrite Province (Coira et al., 1982; de Silva and Francis, Andes are well documented (e.g. Scheuber and Giese, 1999; Scheuber 1991; Trumbull et al., 2006; Salisbury et al., 2011; Freymuth et al., 2015; and Reutter, 1992, Beck et al., 1996; Allmendinger et al., 1997; Kay Brandmeier and Wörner, 2016). The most intense activity produced the and Mpodozis, 2001; Oncken et al., 2006; Hartley et al., 2007; Kley et Altiplano-Puna Volcanic Complex (APVC, de Silva, 1989), a volcano-tec- al., 1999; Kley and Monaldi, 1998) and have been related to the system- tonic province in the Central Andes occupying the high between atic spatio-temporal changes in the geochemical and isotopic composi- 21° and 24°S (Fig. 1). The area of the APVC coincides with the surface tion of erupted lavas during the last 26 M.y. (e.g. Haschke, 2002; Kay et projection of a low-velocity zone, interpreted as a partially-molten al., 2005; Haschke et al., 2006; Mamani et al., 2008, 2010). Volcanic body within the upper crust (~15 to 30 km), the so-called “Altiplano- rocks of earlier stages of Central Andean evolution traversed thin crust Puna Magma Body” (AMPB; Chmielowski et al., 1999; Zandt et al., and are consistently low in Sr/Y, La/Yb, and Sm/Yb ratios, whereas pro- 2003; Ward et al., 2014)(Fig. 1). This body has also been recognized gressively younger magmatic products show increases in the maximum by electrical, gravity, and isostatic anomalies (Schilling et al., 1997; Sr/Y, La/Yb, and Sm/Yb ratios. These changes in the geochemical signa- Haberland and Rietbrock, 2001; Schilling and Partzsch, 2001; Brasse et ture of lavas were attributed to the increasing role of as a stable al., 2002; Schnurr et al., 2007; Prezzi et al., 2009), and is interpreted as residual phase in magma processing within a progressively thicker Cen- an incrementally constructed, upper-crustal (de Silva and tral Andean crust. This is in line with the more radiogenic signatures of Gosnold, 2007; Kern et al., 2016) atop an upper crustal MASH zone the magmatism with time that are attributed to increased crustal assim- (Burns et al., 2015; Ward et al., 2014). ilation (Rogers and Hawkesworth, 1989; Kay and Mpodozis, 2001; Tracking the influence of this partially molten upper crustal batho- Davidson et al., 1990; Haschke, 2002; Haschke et al., 2006; Mamani et lith on magma compositions in arc front is our aim in this al., 2008, 2010). study. The hypothesis is that crustal partial melts will be more

Fig. 1. Global Multi-Resolution Topography image showing location of the volcanic structures (black stars) included in this study: Aucanquilcha (1) – Ollagüe (2) – Uturuncu (3) transect (Michelfelder et al., 2013); San Pedro (4) – Linzor (5) volcanic chain (SPLVC), including La Poruña scorica cone (6), and (7), Cerro del León (8) and Toconce (9) volcanoes (Godoy et al., 2014); Putana (10) – Sairecabur (11) – Licancabur (12) transect, including Purico-Chascón Volcanic Complex (13), and Lascar volcano (14). Dotted areas indicate distribution of Altiplano-Puna Volcanic Complex (APVC, thick) and surface projection of the Altiplano-Puna Magma Body (APMB, thin) (after Zandt et al., 2003). Dashed grey areas indicate extend of joint ambient noise-receiver function S-velocity (Vs) models contours, at 15 km b.s.l., for velocities b3.2 km/s (Ward et al., 2014). Thick lined polygon indicates extend of geological map from Fig. 2. 174 B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186 efficiently mixed and at higher proportions with parental mafic and Francis, 1986; Lazcano et al., 2012; López et al., 2012; Polanco et magmas and therefore have more leverage on the geochemical compo- al., 2012; Silva et al., 2012; López, 2014; Martínez, 2014; Silva, 2015; sition of the erupted lavas than solid crust. Here we use radiogenic iso- Lazcano, 2016). The ~100 ka Chillahuita dome and giant Chao dacitic topes and geochronological data to map the influence of the APMB. To coulée (Guest and Sanchez, 1969; de Silva et al., 1994; Tierney et al., this end we present new geochronological (40Ar/39Ar) and isotopic 2016) are also included in the chain (Fig. 2). Finally, the ~103 ka La (87Sr/86Sr and 143Nd/144Nd) data combined with a compilation of pub- Poruña basaltic- cone is the source of a 8 km long lished geochemical data from the NW-SE San Pedro – Linzor volcanic flow at the far NW end of the SPVC (O'Callaghan and Francis, 1986; chain (SPLVC) in N. Chile (Fig. 1). This 65 km long linear chain strikes Wörner et al., 2000). The SPLVC is underlain mainly by the dacitic at an angle to the general N-S direction of the Central Andes active Sifon Ignimbrite (8.3 Ma, Salisbury et al., 2011) and and front and crosses the western margin of the surficial projection of the volcaniclastics of the 6.5 to 5.6 Ma Toconce Formation, on older (pre- upper crustal APMB (Fig. 1). Neogene) volcanic and volcaniclastic sediments (Ramírez and Huete, 1981; Marinovic and Lahsen, 1984; de Silva, 1989). 2. Geological background 3. Analytical methods Extending from 14°S to 27°, from southern Peru to , the modern of the Central Volcanic Zone is built above where 3.1. Geochronology the subducting plate is dipping at ~30°; the northern and southern limits are where, respectively, the Nazca and Juan Fernandez and Four of our samples were dated by 40Ar/39Ar analyses at the Oregon Nazca ridges are currently subducting at shallow angles (de Silva and State University (OSU) Argon Geochronology Laboratory (USA). The Francis, 1991; Stern, 2004). For this volcanic arc, four main phases of ac- samples were crushed in an iron jaw crusher and then sieved. After- tivity are defined during the eastward migration of the volcanic front wards 200 mg of the 100–500 μm size-fraction of unaltered groundmass (the Andean-Cycle) (e.g. Coira et al., 1982; Scheuber and Giese, 1999; from each sample were hand-picked. Preparation and analyses of sam- Trumbull et al., 2006): 1) a – Upper arc, with effusive ples and standards followed the procedures described in Koppers et al. products mainly erupted in the Coastal Range in Chile; 2) a Mid-Creta- (2003). Fourteen additional samples were prepared for 40Ar/39Ar analy- ceous arc of the Longitudinal and Sierra de Moreno in Chile; 3) ses at the Servicio Nacional de Geología y Minería, Chile aLateCretaceous– arc, and after a period of flat-slab subduc- (SERNAGEOMIN) on and unaltered groundmass. Crushing tion, 4) the – active volcanic arc with eruptive prod- and separation, sample preparation, and analysis were carried ucts mainly located at the Western Cordillera related to steepening (10 out following the procedures and parameters established in Arancibia to 30°) subduction of the below the et al. (2006). (Coira et al., 1982). The study region along the arc front and the APVC is characterized in the past 12 M.y. by (Aitcheson and Forrest, 1994) scattered stratovol- 3.2. and isotope analyses. canoes along the active arc front and on the Altiplano plateau, (Allmendinger et al., 1997) the eruption of large ignimbrite sheet from Thirty-seven samples were crushed in an iron jaw crusher and pow- several , with repeated eruptions, (de Silva et al., 2006, de dered in agate mills. Geochemical and isotopic analyses were carried Silva and Gosnold, 2007; Salisbury et al., 2011; Kern et al., 2016), and out at the GZG (Universität Göttingen, Germany), at the Department (Arancibia et al., 2006)eruptionofyoung(b100 k.y.) rhyodacitic of Geological Sciences, University of Cape Town (UCT; South Africa), domes and coulées (de Silva et al., 1994; Watts et al., 1999; Tierney et and at Activation Laboratories Ltda. (Actlabs; Canada). Procedures for al., 2016). The ignimbrites are mostly “monotonous intermediates” X-ray fluorescence (XRF; major and trace element concentrations) (sensu Hildreth, 1981) dominantly calc-alkaline, high-K dacites to and thermal ionization mass spectrometry (TIMS; 87Sr/86Sr and rhyodacites, with minor . Andesitic bands and andesite inclu- 143Nd/144Nd ratios) analyses at GZG are described in Godoy et al. sions in pumices are observed. The dominant volume of ignimbrites is (2014). Two-sigma analytical errors were b2% for XRF, and b0.004% related to large-scale and structurally-controlled collapse calderas (e.g. for 87Sr/86Sr and 143Nd/144Nd ratios. At UCT samples were analyzed by , Guacha, and ), with significant volumes XRF and Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) (N100 km3) of magma occurrying form “ignimbrite shields” (e.g. Cerro for major and trace elements, following the procedures, standards and Panizos, the Laguna Colorado , and Cerro Purico) (de Silva and parameters detailed in Frimmel et al. (2001). 87Sr/86Sr and Gosnold, 2007; Salisbury et al., 2011). The ignimbrites and domes are 143Nd/144Nd ratios were measured at UCT by NuPlasma HR multi collec- typically crystal-rich (N40 vol%) with of , , tor-ICP-MS (MC-ICP-MS). Sample preparation and equipment condi- , amphibole, and Fe-Ti oxides with occasional sanidine, along tions for these analyses are detailed in Harris et al. (2015). Analytical with ubiquitous , titanite, and (Ort et al., 1996; Lindsay errors (2 S.D.) were b2% for XRF, b3% for ICP-MS and b0.003% for et al., 2001; Schmitt et al., 2001; Grocke et al., 2016), showing a strong 87Sr/86Sr and 143Nd/144Nd ratios. At Actlabs, inductively coupled plas- crustal composition (Lindsay et al., 2001; Schmitt et al., 2001; Kay et ma-optical emission spectrometry (ICP-OES), ICP-MS, and TIMS were al., 2010; Salisbury et al., 2011; Burns et al., 2015; Freymuth et al., utilized for major oxides, trace elements, and 87Sr/86Sr and 2015; Grocke et al., 2016). 143Nd/144Nd analyses, respectively. For ICP-OES and ICP-MS, samples The San Pedro – Linzor volcanic chain (SPLVC) forms a ~65 km long were mixed with a flux of lithium metaborate and lithium tetraborate NW-SE trending lineament of stratovolcanoes between 21°53′S68°23′ and fused in an induction furnace. The melt was immediately poured W and 22°09′S 67°58′W(Figs. 1 and 2). This chain of stratovolcanoes into a solution of 5% nitric acid containing an internal standard, and erupted on the NW margin of the APVC and crosses the western border mixed continuously until completely dissolved (~30 min). Analytical of the APMB (Fig. 1). It includes a series of large and partly complex vol- errors are b2% for each type of analyses. For TIMS, Rb and Sr, and Sm canic edifices (San Pedro – volcanic complex, and Paniri, Cerro and Nd were separated by extraction chromatography. The analyses del León, Toconce, and Linzor volcanoes) consisting of lava, pyroclastic were performed on a Thermo Triton thermal ionization multi-collector and scoria flows and breccias. Petrographically, lava flows of these vol- mass spectrometer. Errors for 87Sr/86Sr and 143Nd/144Nd ratios were canoes vary from basaltic-andesite to -, with b0.004%. For these analyses more information about analytical proce- andesite as the main lithological type. Pyroclastic flows are dacitic, dure, equipment and uncertainties are available at: http://www. while scoria flows and breccias vary from basaltic-andesite to andesite. actlabs.com. Our data are compiled in 1 where the different labo- (Ramírez and Huete, 1981; Marinovic and Lahsen, 1984; O'Callaghan ratories and methods are identified for each sample. B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186 175

Fig. 2. Geological map of a section of the Central Volcanic Zone of the Andes from Aucanquilcha to Licancabur volcanoes (after Tibaldi et al., 2009), with ignimbrite distribution after Salisbury et al. (2011), and geology of Uturuncu volcano after Sparks et al. (2008). Extend of Altiplano-Puna Magma Body (APMB) after Zandt et al. (2003). Volcanoes included in this study on red triangles.

4. Results 4.2. 40Ar/39Ar ages

4.1. Main petrological and geochemical features of the San Pedro-Linzor 40Ar/39Ar results are presented in Tables 2 and 3.Ageswereobtained volcanic chain. by using the Isoplot excel spreadsheet (Ludwig, 2012). Plateau ages were defined as containing N70% of the total 39Ar released. Age plateaus Although some volcanoes from the SPLVC show hydrothermal alter- and inverse isochron ages are in concordance at the 95% confidence ation at their cores and flanks (e.g. Toconce, Cerro del León), analyzed level (Tables 2 and 3). Age spectra and inverse isochron diagrams of rep- samples were obtained from largely unaltered, dense lava flows. This resentative samples are shown in Figs. 5 and 6. Ages obtained on amphi- selection resulted in a bias towards younger lavas. Lavas classify as ba- bole mineral separates show a larger error than those obtained from saltic-andesite to with predominance of dacitic compositions unaltered groundmass (Fig. 5; Table 3). Sample PAE-15 shows no age (Fig. 3; Table 1). Plagioclase and ortho- and clinopyroxene are the plateau and only three steps were used to calculate the inverse isochron main phenocrysts in a consisting of 60 to 75 vol% of plagioclase age (Fig. 5), the age obtained for this sample is therefore not reliable. and pyroxene microlites, and glass (Fig. 4). At Paniri and San Pedro vol- The age obtained for sample PAE-09 was calculated by combining data canoes, have rare phenocrysts (b5 vol%) with skeletal from two analyses, both showing concordant isochron and spectra texture (Fig. 4), while at Toconce and Cerro del León olivine is even ages (Fig. 6; Table 3). scarcer (b2 vol%). Amphibole and biotite are rare, with amphibole Paniri volcano shows the oldest (1.390 ± 0.290 Ma), and the youn- showing disequilibrium textures at the rims. Moreover, resorbed quartz gest (150 ± 6 ka) age for the volcanic chain (Fig. 7). For Cerro del Leon, phenocrysts have been observed in Linzor dacite lavas (Fig. 4). 40Ar/39Ar dating indicates flows with ages of 1.054 ± 0.011 Ma to 275 176 B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186

Table 1 87 86 143 144 SiO2,Na2O, K2O, Sr, Nd, Sr/ Sr, and Nd/ Nd content of analyzed samples from San Pedro – Linzor volcanic chain.

a a a 87 86 143 144 Volcano Sample Latitude Longitude SiO2 Na2O K2O Sr Nd Sr/ Sr Error (2S.D.) Nd/ Nd Error (2.S.D.) (S) (W) (wt%) (wt%) (wt%) (ppm) (ppm) (10−6) (10−6)

La Poruña SP1b 21° 53′ 14″ 68° 29′ 58″ 56.70 3.66 1.72 578 19 0.706630 – 0.512378 – POR-14-01f 21° 53′ 27″ 68° 30′ 39″ 56.21 3.72 1.70 608 – 0.706640 8 0.512393 12 POR-15-02f 21° 53′ 29 68° 29′ 47″ 57.80 3.72 1.83 584 – 0.706265 11 0.512427 11 POR-15-03f 21° 53′ 32″ 68° 29′ 58″ 57.29 3.63 1.76 575 – 0.706353 13 0.512421 13 POR-15-04f 21° 53′ 35″ 68° 30′ 19″ 57.07 3.57 1.77 569 – 0.706272 12 0.512450 12 POR-15-05f 21° 55′ 32″ 68° 34′ 2″ 59.34 3.91 2.10 590 – 0.706184 10 0.512437 11

San Pedro SPP-98-54b 21° 52′ 12″ 68° 29′ 52″ 63.60 4.88 2.72 518 14 0.706660 – 0.512351 – SPP-98-56b 21° 49′ 24″ 68° 27′ 50″ 64.10 4.32 2.86 512 22 0.705710 – 0.512346 – BG-SPL-004e 21° 54 ‘22″ 68° 30′ 3″ 58.50 3.67 1.91 523 20 0.706149 4 –– BG-SPL-010e 21° 50′ 1″ 68° 30′ 1″ 57.70 3.66 1.67 610 20 0.706705 5 –– BG-SPL-015c 21° 53′ 25″ 68° 29′ 33″ 57.40 3.69 1.77 508 17 0.706306 9 0.512404 3 SPSP-14-01f 21° 49′ 55″ 68° 29′ 48″ 62.41 4.37 2.67 584 – 0.706683 1 0.512392 11 SPSP-14-02f 21° 56′ 3″ 68° 30′ 36″ 63.20 4.05 3.16 489 – 0.706414 1 0.512384 12

Paniri BG-SPL-019Ac 22° 1′ 28″ 68° 16′ 6″ 68.80 3.86 3.84 365 29 0.707143 3 0.512347 5 BG-SPL-022c 22° 2′ 48″ 68° 17′ 4″ 56.50 3.47 1.52 663 17 0.706676 6 0.512279 6 BG-SPL-023Ac 21° 59′ 05″ 68° 14′ 45″ 61.60 3.50 2.71 441 25 0.707212 3 0.512338 3 BG-SPL-044Ac 22° 8′ 24″ 68° 16′ 26″ 65.00 3.45 3.61 407 28 0.707253 3 0.512268 4 PANI-12-02f 22° 0′ 55″ 68° 15′ 11″ 65.40 3.97 3.22 432 23 0.70723 15 0.512352 10 PANI-12-07f 22° 3′ 48″ 68° 11′ 46″ 69.70 3.81 3.79 359 25 0.707977 12 0.512317 10 PANI-12-08v 22° 4′ 16″ 68° 11′ 41″ 65.80 3.81 3.03 439 24 0.707577 12 0.512326 7 PANI-12-10f 22° 2′ 49″ 68° 15′ 14″ 67.40 3.93 3.52 421 25 0.706909 13 0.512351 10 PANI-12-14f 22° 7′ 49″ 68° 0′ 11″ 66.10 3.44 3.64 393 26 0.707294 13 0.512366 10 PANI-12-15f 22° 7′ 32″ 68° 16′ 4″ 64.80 3.64 3.39 431 25 0.707318 13 0.512334 9 M28g 22° 6′ 10″ 68° 18′ 12″ 64.40 3.45 3.32 417 28 0.707333 4 0.512339 6

Cerro del Leon BG-SPL-040c 22° 13′ 59″ 68° 14′ 45″ 60.90 3.16 3.02 458 – 0.707875 3 0.512237 4 LEO-10-01c 22° 9′ 30″ 68° 8′ 01″ 63.20 3.39 3.17 408 28 0.707821 3 0.512245 6 LEO-10-02e 22° 9′ 32″ 68° 8′ 01″ 62.60 3.39 2.95 419 28 0.707811 4 –– LEO-10-07e 22° 13′ 46″ 68° 16′ 52″ 60.50 3.46 2.60 464 25 0.707899 4 –– LEO-12-01f 22° 6′ 27″ 68° 7′ 40″ 69.40 4.51 4.29 278 31 0.708045 15 0.512322 8 LEO-12-03f 22° 5′ 54″ 68° 7′ 42″ 68.70 4.45 4.18 319 33 0.708036 12 0.512330 8 LEO-12-04f 22° 5′ 24″ 68° 7′ 44″ 62.80 3.77 2.94 435 29 0.70794 10 0.512313 9 LEO-12-07f 22° 7′ 20″ 68° 7′ 2″ 65.20 2.79 3.46 348 28 0.709573 9 0.512276 9 LEO-12-09f 22° 6′ 57″ 68° 6′ 38″ 67.50 4.02 3.80 319 28 0.70788 11 0.512333 8 LEO-12-C2f 22° 8′ 41″ 68° 4′ 54″ 65.70 3.33 3.46 350 24 0.708334 9 0.512267 9 M25bg 22° 11′ 37″ 68° 10′ 58″ 63.70 3.03 3.32 386 26 0.707765 3 0.512302 11

Toconce BG-SPL-048c 22° 10′ 1″ 68° 3′ 20″ 58.80 3.07 2.11 503 24 0.707693 4 0.512296 10 TOC-10-02e 22° 13′ 17″ 68° 5′ 42″ 63.50 3.35 3.10 391 27 0.708347 3 –– TOC-10-03c 22° 12′ 49″ 68° 5′ 14″ 69.40 3.23 4.42 267 28 0.709346 6 0.512269 20 TOC-10-04c 22° 12′ 49″ 68° 5′ 06″ 64.70 3.21 3.47 338 31 0.708998 1 0.512242 9 TOC-10-08e 22° 14′ 15″ 68° 5′ 24″ 66.80 3.06 3.83 335 30 0.708527 1 –– TOC-12-01f 22° 9′ 3″ 68° 4′ 10″ 68.80 3.25 4.01 292 24 0.708836 12 0.512244 9 TOC-12-02f 22° 9′ 8″ 68° 3′ 57″ 62.90 3.33 2.98 430 26 0.708026 14 0.512293 8 TOC-12-04f 22° 10′ 42″ 68° 4′ 2″ 65.90 3.67 3.24 418 26 0.707848 12 0.512296 11 TOC-12-05f 22° 10′ 42″ 68° 4′ 2″ 66.40 3.65 3.43 390 26 0.707844 14 0.512285 11 TOC-12-10f 22° 15′ 8″ 68° 12′ 54″ 67.30 3.23 4.27 272 27 0.708786 12 0.512286 12 TOC-15-01f 22° 11′ 48″ 68° 3′ 50″ 65.34 3.99 3.16 372 – 0.707601 1 0.512310 10 M21g 22° 14′ 58″ 68° 7′ 49″ 67.60 2.81 4.45 235 29.5 0.708812 3 0.512281 3

Chao Dacite 88054d ––67.90 3.25 3.81 335 – 0.70806 – 0.51224 – Chillahuita 84058d ––68.80 3.42 3.66 325 – 0.70805 – 0.51224 –

a Recalculated 100% free. b Data from Mamani et al. (2010). c Data from Godoy et al. (2014). d Data from de Silva et al. (1994). e Analysis at University of Göttingen (Germany). f Analysis at University of Cape Town (South Africa). g Analysis at ActLabs (Canada).

± 7 ka, while Toconce volcano shows flows ranging in age from 1.294 ± 2010; Table 1), while San Pedro volcano shows values from 0.705710 0.080 Ma to 891 ± 33 ka (Fig. 7). to 0.706705, and from 0.512346 to 0.512404, respectively (compiled from literature data in Mamani et al., 2010; Table 1). Paniri lavas have 4.3. 87Sr/86Sr and 143Nd/144Nd isotope ratios 87Sr/86Sr between 0.706676 and 0.707977, which are significantly higher than values for San Pedro and La Poruña, while the Published and new 87Sr/86Sr and 143Nd/144Nd data, together with 143Nd/144Nd ratios are lower (0.512268 to 0.512366). 87Sr/86Sr isotope

SiO2 (wt%), and Sr and Nd (ppm) contents of lavas erupted in the ratios of Cerro del Leon lavas vary from 0.707811 to 0.709573, while SPLVC are presented in Table 1 including data from La Poruña scoria 87Sr/86Sr isotope ratios of Toconce range between 0.707693 and cone and Chao Dacite and Chillahuita domes. 87Sr/86Sr ratios between 0.709346. 143Nd/144Nd ratios for Cerro del Leon vary between 0.706184 and 0.706640, and 143Nd/144Nd ratios between 0.512378 0.512237 and 0.512333, and for Toconce vary from 0.512242 and and 0.512450 have been obtained for La Poruña lavas (Mamani et al., 0.512310. Both volcanoes exhibit Sr isotope ratios higher than, and B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186 177

Fig. 3. Total-Alkali vs. Silica (TAS) diagram (after Le Maitre, 1984) for analyzed lavas of SPLVC, and Chao Dacite and Chillahuita dacitic domes (Table 1). Lava samples show a well-defined sub-alkaline trend, varying from basaltic-andesite to rhyolitic, with some trachytic composition. Field represents the composition of Central Andes lavas (after Mamani et al., 2010). Segmented line represents subdivision of alkaline and subalkaline lavas (after Irvine and Baragar, 1971). overlapping, those obtained by de Silva et al. (1994) for the Chao Dacite southern flanks of Toconce volcano (N0.9 Ma). This suggests that the and Chillahuita dacitic domes (~0.7081) (Table 1). initial construction of the volcanic edifices along the chain was more or less contemporaneous, between 0.9 and 1.5 Ma. After that, the youn- 5. Discussion ger parts of the edifices formed progressively north-westwards. Thus, the youngest lavas dated for the volcanic chain correspond to Paniri vol- 5.1. Age relations cano, with 164 ± 3 and 150 ± 6 ka, respectively, and at Cerro del Leon, with 275 ± 7 ka. For San Pedro volcano, historical activity and New 40Ar/39Ar ages (Tables 2 and 3) and published age data (Table emissions have been reported (Global Volcanism Program, 2013). 4) show an increase in age for the SPLVC from the NW towards the SE O'Callaghan and Francis (1986) suggested that this volcano is younger in parallel to the increase in Sr-isotope ratios (Fig. 7). At the SPLVC the than San Pablo, which pre-dated the last glacial episode. Moreover, a oldest lavas correspond to a series of flows at the base of Paniri volcano 40Ar/39Ar age of 107 ± 12 ka was obtained for the southern lava flow (1.390 ± 0.290 Ma), at the base (1.054 ± 0.011 Ma) and the southern of the volcano (Delunel et al., 2016). Thus, a Pre-Holocene to Recent flank (0.913 ± 0.080 Ma) of Cerro del Leon volcano, and at the lower age is proposed for this volcano (Fig. 7). On the other hand, the eruption

Fig. 4. Photomicrographs showing typical textures of lavas from the SPLVC. a) Skeletal olivine (Ol) in a plagioclase + glass groundmass (Grd) from Paniri volcano (sample BG-SPL-022, 10×). b) Clinopyroxene (Cpx), orthopyroxene (Opx), and plagioclase (Plg) in plagioclase + glass groundmass (Grd) from San Pedro volcano (sample BG-SPL-015, 2×). c) Orthopyroxene (Opx) and plagioclase (Plg) in a glassy groundmass (Grd) from Toconce volcano (sample TOC-10-04, 2×). d) Embayed quartz (Qz), and plagioclase (Plg), in a plagioclase + glass groundmass (Grd), lava from Linzor volcano (BG-SPL-030, 2×). All bars indicate 1 mm length. 178 B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186

Table 2 40Ar/39Ar from incremental heating analyzed lava samples at OSU Argon Geochronology Lab (USA).

Age spectrum Inverse isochron analyses

Sample Latitude Longitude Sample description Phaseb Age Error 39Ar ncaAge Error nc 40Ar/36Ar Error a (S) (W) (Ma) (2σ) (%) (Ma) (2σ) intercept (2σ)

BG-SPL-022 22° 2′ 48″ 68° 17′ 4″ Western flank of Paniri. andesitic flow gm 0.402 0.460 95.8 9/11 0.03 0.380 0.560 9/11 297.0 13.0 0.03 BG-SPL-040 22° 13′ 59″ 68° 14′ 45″ S flank of C. del Leon, dacitic flow gm 0.913 0.080 93.8 10/12 0.24 0.890 0.330 10/12 294.9 4.2 0.26 TOC-10-04 22° 12′ 49″ 68° 5′ 6″ Upper S flank of Toconce, dacitic flow gm 0.891 0.033 100.0 12/12 0.08 0.891 0.037 12/12 295.5 1.6 0.09 TOC-10-09 22° 14′ 55″ 68° 5′ 51″ S flank of Toconce, andesitic flow gm 1.294 0.080 100.0 13/13 0.19 1.240 0.360 13/13 296.0 2.7 0.20

a MSWD – mean square of weighted deviates. Preferred ages are in bold. b Abbreviation: gm – groundmass. c Number of data points used in plateau and isochron calculations; each step heating represents one data point. of the mafic lava in the area at La Poruña scoria cone, at the NW end of (e. g. Uturuncu, Licancabur, Aucanquilcha, Ollagüe, and the active Lascar the chain, occurred at ~100 ka (3He exposure age; Wörner et al., 2000) volcano, Gardeweg et al., 2011), in the last 2 M.y. cannot be related to contemporaneous with the formation of the silicic domes (Chao and (1) differences in the composition of the underlying crust, (2) increased Chillahuita) in the center and the SE end of the chain (ages in Tierney assimilation with time in a thickening crust or (3) during thermal evo- et al., 2016; Fig. 7). lution of a MASH system. In summary, published K/Ar, 3He, as well as published and new 40Ar/39Ar geochronological data indicate that the SPLVC evolved during 5.2. Sr- and Nd-isotope variation by different degree of crustal assimilation? the last 2 Ma with activity continuing into the Holocene at San Pedro, Paniri and possibly Cerro de Leon volcano, based on morphological ob- Fig. 9 shows 143Nd/144Nd vs. 87Sr/86Sr isotope compositions in lavas servations. These are consistent with the age data, suggesting a from along the SPLVC, together with selected literature data from volca- younging of activity, in the last 1 M.y., along the chain from SE to NW, noes at different position with respect to the border of the APMB (Feeley i.e. from Toconce to San Pedro (Fig. 7). Contemporaneous eruption of and Davidson, 1994; Matthews et al., 1994; Figueroa et al., 2009; basaltic-andesite and siliceous magmas, however, occurred between Mamani et al., 2010; Walker, 2011; Michelfelder et al., 2013). The NW 80 and 110 ka with La Poruña scoria cone, and Chao and Chillahuita to SE isotopic variation between Aucanquilcha, Ollagüe, and Uturuncu domes along the entire chain. volcanoes follow the same isotopic trend as that from S to N from Lascar Within this relatively short time-window (1.3 Ma) Sr- show volcano to the Putana-Sairecabur-Licancabur volcanic chain (Fig. 1)and no correlation with age (Fig. 8). This is different on a local scale for the along the NE-SW- trending SPLVC. The most “crustal” apex of the trends evolution of individual stratovolcanoes, e.g. the Purico – Chascon volca- corresponds to Uturuncu volcano (Fig. 9) which also lies within the cen- nic complex (Burns et al., 2015) and Aucanquilcha Volcanic Cluster tral part of the APMB with lowest S-wave seismic velocities of b2.1 km/s (Grunder et al., 2006; Klemetti and Grunder, 2008; Walker, 2011; (Ward et al., 2014; Fig. 1). Less crustal isotopic signatures then correlate Walker et al., 2013), where decreasing on Sr-isotopes indicate waning with higher seismic velocities as the transects are crossing the margin of of the magmatic systems (Fig. 1). Moreover, temporal shifts in isotopic the partially molten APMB zone (Fig. 9). composition of magmas observed on the larger temporal (N10 Ma) As a negative linear correlation exists between Sr and Nd isotopic and spatial (N100 km) scale in the Central Andes in general (e.g. systems, increasing Sr-ratios while Nd ratios decrease, we focus on Sr- Mamani et al., 2010 and reference therein) are related to crustal thick- isotope characteristics of the SPLVC, related to the degree of crustal con- ening (McMillan et al., 1993; Haschke, 2002; Haschke et al., 2006)and tamination. In this case, the SPLVC shows a southeastward increase of composition (Wörner et al., 1992; Mamani et al., 2010). Thus, in con- 87Sr/86Sr with decreasing Sr concentration (Fig. 10a). This observation trast to the general evolution across the Central Andes, isotopic shifts is typical for many but not all composite cones in the Andean CVZ at the SPLVC, and the other volcanoes selected here for comparison (Davidson et al., 1990) and can be interpreted to reflect increasing

Table 3 40Ar/39Ar from incremental heating analyzed lava samples at SERNAGEOMIN (Chile).

Age spectrum Inverse isochron analyses

Sample Latitude Longitude Sample description Phaseb Age Error 39Ar nca Age Error nc 40Ar/36Ar Error a (S) (W) (Ma) (2σ) (%) (Ma) (2σ) intercept (2σ) PAE-02 22° 2′ 47″ 68° 12′ 35″ N flank of Paniri, dacitic flow amph 0.264 0.099 100.0 8/8 0.67 0.240 0.160 7/8 296.9 4.4 0.71 PAE-03 22° 2′ 50″ 68° 12′ 29″ N flank of Paniri, dacitic flow gm 0.325 0.008 100.0 8/8 0.45 0.323 0.012 8/8 295.8 3.0 0.50 PAE-08 22° 6′ 13″ 68° 17′ 49″ SW flank of Paniri, andesitic flow amph 0.150 0.006 100.0 8/8 1.40 0.151 0.007 7/8 292.5 6.6 1.18 PAE-090 22° 0′ 0″ 68° 14′ 8″ N flank of Paniri, dacitic flow amph –––––1.390 0.290 13/15 297.0 4.6 0.49 PAE-091 22° 0′ 0″ 68° 14′ 8″ N flank of Paniri, dacitic flow amph 0.980 0.360 100.0 8/8 0.97 0.970 0.460 8/8 297.0 11.0 1.12 PAE-092 22° 0′ 0″ 68° 14′ 8″ N flank of Paniri, dacitic flow amph 1.420 0.300 100.0 7/7 0.72 1.370 0.340 7/7 297.6 5.0 0.75 PAE-15 22° 6′ 48″ 68° 7′ 12″ W flank of C. del Leon, dacitic flow gm –––––1.137 0.051d 3/8 295.7 8.9 0.17 PAE-16 22° 3′ 42″ 68° 6′ 42″ NE flank of C. del Leon, trachy-dacitic flow gm 1.054 0.011 76.7 5/8 0.64 1.037 0.032 7/8 297.4 3.5 0.70 PAE-25 22° 3′ 44″ 68° 15′ 31″ E flank of Paniri, trachy-dacitic flow gm 0.164 0.003 100.0 8/8 0.76 0.163 0.003 8/8 295.6 4.1 0.90 PAE-36 22° 11′ 49″ 68° 8′ 33″ S flank of C. del Leon, andesitic flow gm 0.367 0.018 82.5 3/8 0.54 0.334 0.055 8/8 297.1 2.4 0.50 PAE-37 22° 10′ 0″ 68° 7′ 2″ SW flank of C. del Leon, andesitic flow gm 0.664 0.012 93.8 7/8 0.16 0.687 0.019 8/8 292.5 1.7 1.7 PAE-42 22° 14′ 37″ 68° 7′ 30″ SW flank of Toconce, dacitic flow gm 0.959 0.005 100.0 8/8 0.68 0.960 0.005 8/8 294.5 1.7 0.55 PAE-43 22° 7′ 55″ 68° 15′ 16″ S flank of Paniri, andesitic flow amph 0.625 0.093 100.0 8/8 0.24 0.610 0.110 8/8 298.1 6.4 0.16 PAE-44 22° 11′ 28″ 68° 10′ 55″ SW flank of C. del Leon, andesitic flow gm 0.628 0.007 97.9 7/8 0.23 0.623 0.008 8/8 298.5 2.0 0.39 PAE-48 22° 9′ 31″ 68° 4′ 55″ SE flank of C. del Leon, dacitic flow gm 0.275 0.007 100.0 8/8 0.05 0.275 0.008 8/8 295.3 2.1 0.05 PAE-55 22° 3′ 45″ 68° 11′ 24″ E flank of Paniri, dacitic flow amph 0.640 0.140 97.2 5/7 0.06 0.650 0.200 5/7 291.0 28.0 0.05

PAE-090 as result of combined analyses of PAR-091 and PAE-092. a MSWD - mean square of weighted deviates. Preferred ages are in bold. b Abbreviation: gm – groundmass. c Number of data points used in plateau and isochron calculations; each step heating represents one data point. d Not reliable, see text for discussion. B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186 179

Fig. 5. Age spectra and inverse isochron diagrams for representative samples of dated amphibole (PAE-02) and groundmass (PAE-03, PAE-15 and TOC-10-09). Age diagrams for sample 40 39 40 39 PAE-15 shows no plateau and thus the age from this sample is not reliable. Box heights are 2σ error. Analytical error ellipses in isochron diagrams and initial Ar/ Ar ( Ar/ Ari) are at the 2σ level. Light blue data indicate rejected analyses. MSWD = mean standard weighted deviates. 180 B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186

Fig. 6. Age spectra and inverse isochron diagram of two analyses for sample PAE-09 (amphibole). Combined isochron diagram for sample PAE-09 is also shown. Boxheightsare2σ error 40 39 40 39 (2 s). Analytical error ellipses in isochron diagrams and initial Ar/ Ar ( Ar/ Ari) are at the 2σ level. Light blue data indicate rejected analyses. MSWD = mean standard weighted deviates. B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186 181

Fig. 7. Satellite image of the SPLVC showing the spatial distribution of 87Sr/86Sr isotopic ratios presented on Table 1. Sr-isotope ratios decrease from SE (Toconce volcano) to NW (San Pedro volcano). Inset shows distribution of ages, with errors (2σ). Bar for ages at San Pedro after O'Callaghan and Francis (1986). proportions of assimilated crustal rocks in magmas together with higher erupted along the SPLVC do not have heavy rare earth element-depleted degrees of low-pressure differentiation. As volcanoes of the SPLVC are trace element patterns that would be indicative of magma evolution located entirely in the crustal domain (sensu Mamani et al., under high pressure (i.e. involving garnet as a residual phase), even 2010) parental magmas that ascend from the lower crust should have though the crust was undoubtedly thick (N60 km) when these lavas similar isotopic characteristics (Godoy et al., 2014). Models for further were erupted. HREE-depleted patterns, however, do occur in many evolution of these parental magmas involve two fun- CVZ lavas erupted (b10 Ma) after the last main phase of crustal thicken- damentally different, although not mutually exclusive processes of in- ing of the Central Andes (Mamani et al., 2010). As argued by Godoy et al. teraction between parental magmas and crustal rocks: (Aitcheson and (2014), the absence of a deep-crustal geochemical signature for lavas Forrest, 1994) Assimilation of shallow crustal material during magmatic from SPLVC suggests that magma genesis is dominated by shallow as- differentiation mainly by fractional crystallization (the “classic” AFC similation of crustal melts that are derived from the APMB. Thus, lavas process, e.g. Davidson et al., 1990; Feeley and Davidson, 1994; Caffe et erupted at this volcanic chain evolved and assimilated crustal material al., 2002), and (Allmendinger et al., 1997) wholesale mixing between at shallow levels (Godoy et al., 2014; Martínez, 2014). “mashed” parental magmas and crustal melts that may either be de- Taking into account considerations from other petrologic studies of rived from melting at deep or shallow crustal levels (e.g. Blum-Oeste Central Andes volcanism (e.g. Davidson et al., 1990; Caffe et al., 2002; and Wörner, 2016). Godoy et al. (2014) have pointed out that magmas Kay et al., 2010), simple AFC models (DePaolo, 1981)wereusedto

Table 4 Published 3He, K/Ar and 40Ar/39Ar ages for San Pedro – Linzor volcanic chain.

Sample Latitude Longitude Age error Method Observations Reference (S) (W) (Ma) (2σ)

POR-02 21° 53′ 5″ 68° 30′ 0″ 0.103 0.001 3He Lava flow from La Poruña Wörner et al. (2000) SP12–02A 21° 56′ 2″ 68° 30′ 36″ 0.107 0.012 40Ar/39Ar Lava flow from the SW dome from San Pedro volcano Delunel et al. (2016) ZZ-06 22° 12′ 53″ 68° 2′ 26″ 1.70 0.20 K/Ar Outcrop on slope of the Toconce volcano Seelenfreund et al. (2009) ZZ-11 22° 12′ 54″ 68° 2′ 17″ 1.10 0.20 K/Ar Outcrop on slope of the Toconce volcano Seelenfreund et al. (2009) ZZ-27a 22° 8′ 30″ 68° 16′ 3″ 0.50 0.10 K/Ar Andesitic flow of the ower slope of the Paniri volcano Seelenfreund et al. (2009) ZZ-42 22° 5′ 19″ 68° 11′ 48″ 0.30 0.10 K/Ar Andesitic flow at the upper slope of the Paniri volcano Seelenfreund et al. (2009) ZZ-46 22° 8′ 30″ 68° 16′ 3″ 0.40 0.10 K/Ar Andesitic flow of the ower slope of the Paniri volcano Seelenfreund et al. (2009) A88-15 22° 15′ 15″ 68° 9′ 15″ 1.10 0.10 K/Ar Southern dacitic flow from Toconce volcano Baker and Francis (1978) 182 B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186

Fig. 8. 87Sr/86Sr vs. Age (ka) diagram showing the absence of a clear relationship between the Sr isotope composition and age. For SPLVC isotopic data from samples SPSP-14-02 (San Pedro), M28 (Paniri), M25 (Cerro del Leon), and M21 (Toconce) (Table 1) correspond to geochronological analyses SP12-02A (Delunel et al., 2016), and PAE-08, PAE44 and PAE 42 (Table 3), respectively. Bars indicate 2σ error. simulate the composition of SPLVC lavas, constraining the amount of Results of RAFT-modeling (Aitcheson and Forrest, 1994; Table 6)in- crustal material using Eq. (5) from the RAFT model by Aitcheson and dicate that the proportion of assimilated crust varies from ~12% to ~31%. Forrest (1994). As an initial composition, we consider the basaltic-an- For Toconce and Cerro del Leon, magma assimilation of crustal compo- desite (BA) end-member proposed as parent magmas of the Central nents varies between ~23% to ~31%. Lavas from Paniri assimilated be- Andes (Blum-Oeste and Wörner, 2016). Thus, a sample from Lascar vol- tween 12% and 23% of crustal material, while the calculated cano (Table 5) was selected. This sample shows low Sr-isotope ratio proportion of assimilated crustal material is lower at San Pedro volcano (~0.7057) (Matthews et al., 1994) corresponding to the isotopic base- and La Poruña scoria cone, reaching up to ~13%. Thus, decreasing assim- line values of MASH-magmas derived from the lower crust (0.705; ilation is observed from NW to SE along the SPLVC (Fig. 10a). sensu Davidson et al., 1990). Moreover, the selected sample has a low

SiO2 (~57 wt%) and Mg number (defined as 100MgO/(MgO + FeO)) 5.3. What is the role of the Altiplano-Puna Magmatic Body? in mole per cent) (Mg# = 51), similar to the BA end-member (Blum- Oeste and Wörner, 2016). As the contaminant, samples from the Paleo- When we combine our new isotope and age data with previously zoic Andean basement we used a bulk upper crustal composition published data in the region, systematic shifts in isotopic composition of the zone that correspond to the northern Sierra de Moreno are recognized for lavas erupted during the past b2Maalongatransect (Lucassen et al., 2001), which is exposed 60 NE of the SPLVC. Crustal that crosses the western margin of the APMB (Fig. 10b). We will now rocks from this area have 87Sr/86Sr ratios ranging from 0.707 and consider the hypothesis that magmas maybe variably influenced by a

0.728, with SiO2 values from 54 to 69 wt%, and Mg#·between 35 and crustal component derived from the APMB. The increase in radiogenic 60 (Lucassen et al., 2001). For AFC-model calculations, the average Sr Sr observed in the NW-SE SPLVC (our data) towards the APMB is consis- composition of crust was used. Also, a mineral assemblage tent with Sr isotope variations along transects from Ollagüe, and was generated taking into account the petrographic characteristic of Aucanquilcha volcanoes to Uturuncu (Michelfelder et al., 2013)and the lavas from the San Pedro – Linzor volcanic chain (O'Callaghan and (in a S to N direction) from Lascar volcano to Licancabur, Putana, and Francis, 1986; Godoy et al., 2014; López, 2014; Martínez, 2014; Silva, Sairecabur (using data compilation by Mamani et al., 2010; Fig. 10b). 2015; Lazcano, 2016)(Table 5). The resulting models are plotted in Uturuncu volcano is located close to the center of the APMB (Fig. 1) Fig. 10a. and has the highest 87Sr/86Sr, and lowest 143Nd/144Nd ratios (Muir et al.,

Fig. 9. 143Nd/144Nd vs. 87Sr/86Sr diagram for selected volcanic centres in the Central Andes. Dotted lines indicate joint ambient noise-receiver function inversion S-velocity models contours

(Vs), at 15 km b.s.l., where seismic velocities b 2.1 km/s are indicative of the Altiplano-Puna magmatic body (APMB) (Ward et al., 2014). Square area represents values for ignimbrites of the Altiplano-Puna Volcanic Complex (data from Kay et al., 2010; Burns et al., 2015). B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186 183

Fig. 10. 87Sr/86Sr vs Sr (ppm) diagram for a) analyzed samples from SPLVC (Table 1), and b) selected lavas erupted in the Central Andes. In a) white arrows represent proposed trends for closed system fractional crystallization starting from magmas that were initially formed by an AFC process (arrows widths are 0.002 on the 87Sr/86Sr ratio). Plotted AFC models according to data from Table 5. Numbers in italic indicate estimated remaining melt fraction (F, %). In b) lavas from Uturuncu lavas show a trend similar to a magma mixing model (inset), while those of Aucanquilcha and Lascar volcanoes show an almost horizontal trend similar to fractional crystallization (F.C.) models (inset). AFC in the inset corresponds to a plagioclase-dominated assimilation and fractional crystallization trend. Square area represents values for ignimbrites of the Altiplano-Puna Volcanic Complex (data from de Silva et al., 1994; Lindsay et al., 2001; Kay et al., 2010; Burns et al., 2015).

2014, 2015) of any andesitic magma in the active volcanic front (Fig. 9). volcanoes (Michelfelder et al., 2013) maximum Sr isotope ratios drop The high Sr isotope ratios of Uturuncu andesites were related to mixing to 0.706, i.e. typical values for the volcanoes of the active front and be- between mafic magmas and dacite magma derived from the APMB by yond the western border of the APMB (Fig. 11). A similar compositional interaction within a ~11 km thick vertical mush column (Muir et al., change with increasingly radiogenic Sr isotope signatures is observed in 2014, 2015). The radiogenic Sr and Nd isotope values of Uturuncu lavas along S-N transect from Lascar to Licancabur, Sairecabur and match those of the evolved, large volume ignimbrites in the southern Putana (Figs. 9 and 10). In essence, volcanoes that are located close to, CVZ (Fig. 9) and suggest large crustal contributions (Muir et al., 2014, or outside the margins of the APMB (e.g. San Pedro, Aucanquilcha, Las- 2015) that are similar to the crustal components of up to 60% have car volcanoes, La Poruña scoria cone), show consistently lower 87Sr/86Sr been proposed for APVC ignimbrites (Freymuth et al., 2015). In a tran- ratios, and higher 143Nd/144Nd ratios (Fig. 9), even in lavas with N65 wt% sect from Uturuncu to the west towards Ollagüe, and Aucanquilcha SiO2. 184 B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186

Table 5 AFC-type model parameters (after DePaolo, 1981) for erupted magmas at San Pedro – Linzor volcanic chain. Bulk D according to partitioning coefficients from Rollinson (1993) for basaltic melts.a

Initial Contaminant Mineral (% assemblage vol)

Location Lascar volcano Sierra de Moreno Plagioclase 40 Reference Matthews et al. Lucassen et al. Clinopyroxene 30 (1994) (2001) Sample LA 123 3/291 4/316 Orthopyroxene 15 a SiO2 (wt%) 57.55 68.80 65.28 Hornblende 5

Al2O3 17.10 13.41 15.12 Olivine 5 (wt%)a CaO (wt%)a 7.11 2.72 1.78 Sr Na2O 3.64 3.20 2.47 D (bulk) 0.84 (wt%)a a Nd K2O (wt%) 1.55 1.64 3.19 D (bulk) 0.2 MgO (wt%)a 3.78 1.94 2.14 FeOt (wt%)a 6.36 5.93 5.63 Sr (ppm) 711 271 185 87Sr/86Sr 0.705765 0.721843 0.72777 Conditions

Nd (ppm) 25 27 37 r = Ma/Mc 0.6 143Nd/144Nd 0.51247 0.511961 0.512087 t=totalFeasFe2+. a Recalculated 100% water free.

We propose that lateral variations in Sr- and Nd-isotope composi- tions of magmas erupted across the margin of the APMB are related to increased degrees of assimilation by this magmatic crustal body (Fig. 11), rather than by vertical or lateral heterogeneity of the crust. S- wave velocities indicate an increase in melt/fluid percentage from the Fig. 11. Schematic cross section showing variation of crustal contamination at the margin of the partially molten APMB from ~4% for zones with S-veloci- Altiplano-Puna Volcanic Complex with cross-sections of the joint ambient noise-receiver ties of 3.2 km/s, to ~10% (2.9 km/s), and up to 25% in zones with veloc- function inversion S-velocity model from the C-C′ profile by Ward et al. (2014).Velocity ities b1.9 km/s (Figs. 1, 9, 11; Schilling et al., 1997; Zandt et al., 2003; contour lines of 3.2, 2.9, 2.5, and 2.1 km/s are shown. Magmatism closer to the APMB core with increasing melt/fluid percentages favors the interaction between mafic Ward et al., 2014). Such differences in melt proportion within the magmas and the upper crustal mush zone. At the center of the APMB, andesite magmas APMB should result in variable degrees of interaction between ascend- have 87Sr/86Sr ratios N0.710 (e.g. Uturuncu volcano) representing the highest proportion ing magmas coming from deeper sources and shallow partial crustal of crustal melts. Dark grey areas represent primary mafic magmas ascending from melts. Interaction between less differentiated magma and this crystal- deeper sources. Shades of grey indicate extend of crustal contamination based on 87Sr/86Sr data. Upper magmatic chamber location according to data from Martínez rich mush increases from the border to the center of the mush-type (2014) and Muir et al. (2014). zone as observed along the SPLVC (increasing from ~12 to ~31% assim- ilated crustal material; Table 6). At the center of the APMB, where as- similation is most significant (Fig. 11) magmas interact along the 6. Conclusions entire mush column (Muir et al., 2014, 2015) and become more assim- ilated by crustal melts. Towards the margin, less radiogenic arc magmas The SPLVC has grown and evolved over the past 2 Ma, during the (e.g. Lascar and Aucanquilcha volcanoes) ascend from their deep source waning of the ignimbrite flare-up in the APVC. The chain developed to the surface with increasingly less crustal interaction with the APMB across the western margin of the APMB, an upper crustal, partially mol- (Fig. 11, Matthews et al., 1994; Walker et al., 2013). ten zone with decreasing proportion of crustal melts towards its mar- gins. Arc magmas that interacted with the lower crust prior to their ascent mixed with different proportions this upper-crustal melt zone (APMB) to explain the formation of magmas with variable radiogenic Sr and Nd isotope compositions. Similar increasing Sr-isotope ratios of between 0.709 and 0.707 are detected in transects across the margins Table 6 of the APMB reaching maximum values (0.710–0.717) in lavas of Calculated ρ and assimilated crust calculated using Eq. (5) by Aitcheson and Forrest (1994). Sr (ppm) and 87Sr/86Sr data according to analytical results (Table 1), and estimat- Uturuncu volcano above the center of this partially molten zone. Volca- ed remaining melt fraction (F) from AFC-type model using sample 4/316 (Table 4). noes located outside the limits of the APMB (e.g. San Pedro-San Pablo, Aucanchilca, Lascar), are less radiogenic in Sr and similar to CVZ F (%) Sr 87Sr/86Sr r (crust/magma Assimilated crust (ppm) ratio) (%) magmatism elsewhere. The lower degree of interaction with APMB crustal components from its transitional crystal-rich and cooler mush 90 601 0.70673 0.15 13.0 zones in the marginal of the APMB could explain the volcanoes with 85 550 0.707302 0.23 18.4 80 502 0.707948 0.30 23.1 less radiogenic Sr isotope compositions (e.g. SPLVC, Aucanquilcha, 75 458 0.708675 0.37 27.2 Ollagüe). This behavior is independent of time as observed at 70 416 0.709495 0.45 31.0 Aucanquilcha and Lascar volcanoes which show similar low radiogenic signatures at different ages (Fig. 8). This indicates that the presence of Sample fl BG-SPL-022a 663 0.706676 0.15 12.7 the APMB has in uenced the composition of erupted magmas for at BG-SPL-010 (San Pedro)b 610 0.706705 0.14 12.2 least the last 2 M.y. POR 14 01 (La Poruña)b 608 0.706640 0.14 12.2 Essentially, there is a direct correlation between the Sr and Nd isoto- a Bulk partitioning (DSr) = 0.84. pic composition of erupted lavas at various stratovolcanoes and the b Bulk partitioning (DSr) = 1.21. AMPB seismic velocity structure within the crust that underlies them B. Godoy et al. / Journal of Volcanology and Geothermal Research 341 (2017) 172–186 185

(Figs. 9, 11). We consider this the strongest evidence for the fundamen- Frimmel, H.E., Zartman, R.E., Späth, A., 2001. Dating continental break-up in the Richtersveld Igneous Complex, South Africa. J. Geol. 109 (4):493–508. http:// tal role that the partial molten zone of the APMB excerts on the isotopic dx.doi.org/10.1086/320795. signature (and thus degree of crustal component) of erupted magmas Gardeweg, M., Amigo, A., Matthews, S.J., Sparks, R.S.J., Clavero, J., 2011. Geología del along the arc front volcanoes. We also note the near absence of the Volcán Láscar, Región de Antofagasta. Carta Geológica de Chile, Serie Geología Básica, No 131, escala 1:50000. 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